1. Field of the Invention
This invention relates generally to a ranging receiver and more particularly to a ranging receiver that utilizes chirp ranging signals.
2. Background Information
Ranging systems are used to determine the location or global position of one or more objects relative to one or more transmitters. Radar systems and Global Navigation Satellite Systems (GNSS) are two examples of well known ranging systems.
GNSS receivers determine their global positions based on the time delays associated with code and carrier signals they receive from associated satellites. The GNSS receivers operate in known manners to align locally generated versions of the codes and carriers with the received signal based on correlation measurements. The GNSS receivers then determine the time delay between the known transmission time of the signal and the time of the receipt of the signal based on the phases of the local codes and carriers, and calculate pseudoranges to the respective satellites based on the associated time delays. A global position is determined in a known manner using the pseudoranges to three or more satellites. A given pseudorange is computed from the difference between the presumed time of code transmission by the satellite and the time of receipt of the code at the receiver, multiplied by the speed of light. The pseudorange value thus contains the actual physical range to the satellite in addition to the clock errors at both the satellite and receiver. In GNSS systems, operators of ground control networks continually estimate the clock drifts of the satellites and provide these data to the receivers as part of real time kinematic (RTK) or other broadcast data messages. Further, the GNSS receiver processing software operating in a known manner can compute the position of the receiver as well as the receiver clock errors, provided the receiver has sufficient numbers of measurements, and the calculated position is thus corrected for both satellite and receiver clock errors.
The receiver receives not only line-of-sight, or direct path, satellite signals but also multipath signals that are reflected to the receiver from the ground, bodies of water, nearby buildings, and so forth. The multipath signals, which arrive at the receiver slightly later than the direct-path signal, combine with the direct-path signal to produce a distorted received signal. The distortion of the received signal adversely affects code and, to lesser degree, carrier alignment operations since the correlation measurements are made using the received signal—including the multipath components thereof. The distortion may be such that the receiver attempts to align to a multipath signal instead of the direct-path signal. This is particularly true for multipath signals that arrive at the receiver close in time to the receipt of the corresponding direct path signal.
One way to more accurately align the received and the locally-generated PRN codes is to use the “narrow correlators” discussed in U.S. Pat. Nos. 5,101,416; 5,390,207 and 5,495,499. It has been determined that narrowing the delay spacing between early and late correlation measurements substantially reduces the adverse effects of noise and multipath signal distortion on the early-minus-late measurements. The delay spacing is narrowed such that the noise correlates in the early and late correlation measurements. Also, the narrow correlators are essentially spaced closer to a correlation peak that is associated with the punctual PRN code correlation measurements than the contributions of many of the multipath signals. Accordingly, the early-minus-late correlation measurements made by these correlators are significantly less distorted than they would be if they were made at a greater interval around the peak.
Another way to more accurately align the received and the locally-generated PRN codes is to use a multipath mitigation processing technique that iteratively produces estimates of the direct path signal and one or more of the multipath signals. One such technique is described in U.S. Pat. Nos. 5,615,232 and 6,692,008. Another technique that uses multiple correlators is described in U.S. Pat. No. 5,414,729. Yet another multipath mitigation technique is described in U.S. Pat. No. 7,738,536.
Note that all GNSS methods of multipath mitigation are limited by the broadcast bandwidth of these systems. The limit of the GNSS multipath mitigation techniques to separate a multipath signal from a direct path signal utilizing a 20 MHz broadcast bandwith and signal processing is about 4 meters. In other words, if the multipath signal overlap of the direct path signal is within 4 meters, the mitigation techniques cannot clearly distinguish the direct path signal from the combined signal and a corrupted tracking error may result. It is well known that the use of wider band systems, such as Ultra Wide Band systems that have much wider bandwidths then GNSS, can support multipath mitigation techniques that can discern the difference between the direct signal and the multipath signal when the two are closer together. For example, a system utilizing a 6 GHz Ultra Wide Band signal should theoretically be 300 times more accurate than one utilizing a 20 MHz GNSS signal.
In certain systems pseudolites are utilized to provide additional ranging signals, particularly in environments in which the pseudolites can be placed to essentially avoid certain reflectors, such as particular buildings and so forth, and/or in environments in which portions of the view of the sky may be blocked by buildings and so forth. The pseudolites are ground-based transmitters that transmit ranging signals, such as GNSS-like signals containing PRN codes. The pseudolite signals, like the GNSS signals, are reflected from reflectors that are nearby the antenna, such as the ground, the antenna frame and so forth, and thus, multipath mitigation techniques may be required for the pseudolite signals as well.
The multipath techniques described above work well, and the systems can obtain centimeter accuracies for clock phase measurements in environments in which the multipath signals arrive relatively close in time to the direct path signals, i.e., the multipath signal and the direct path signal are separated by about 4 meters. However, multipath signals that are closer than 4 meters to the direct path signal, that is, multipath signals that received within nanoseconds of the direct path signal, continue to be sources of error. Environments in which such errors may occur are, for example, construction sites in which a GNSS receiver may be in use in an excavation cavity with contours that act as nearby signal reflectors for both GNSS satellite signals and pseudolite signals.
Accordingly, there remains a need for a ranging receiver that can provide for even greater accuracy in situations in which multipath signals arrive at the receiver antenna particularly close in time to the direct path ranging signals.